Selecting Background Image :
The program has several background images. you can change it in the "preferences panel" (menu: "rs-16x" > "Preferences...").

Using Your Own Image :
Any image file can be used as the backgroung image by placing it into the "Background Images" folder (~/Library/Application Support/RS-16X/Background Images"/). The size of a image shuld be "800 Pixel x 561 pixel" or larger.